Computer disc drives typically use thin film media to store information in a high-density, high-reliability format. Thin film media can accommodate a high storage density, but thin films such as cobalt alloy films are significantly less durable than films composed of the magnetic and alumina particles used in particulate media. The durability of the disc is enhanced by application of a protective layer of very hard material over the cobalt alloy film. A typical protective layer is an overcoat of sputtered amorphous carbon about 100 .ANG.to 250 .ANG. thick. The amorphous carbon overcoat structure can be thought of as a hybrid between diamond and graphite. Other materials which have been used for overcoats include sputtered ceramic zirconium oxide and amorphous films of silicon dioxide.
The overcoat surface is usually lubricated to further reduce wear of the disc due to contact with the magnetic head assembly. Lubricant is typically applied evenly over the disc in a molecularly thin film having a thickness from 10 to 50 .ANG.. Thicker films tend to be spun off by centrifugal forces. Slider-disc interaction, air shear and evaporation may also affect the amount of lubricant on a disc.
Disc surfaces have been textured to affect the tribology between the slider and the disc. A surface roughness of 50 to 350 .ANG. peak-to-valley is typical. The texturing can be provided by texturing of any of the disc layers. For instance, the overcoat layer may be applied over a smooth surfaced magnetic layer, with the overcoat layer being mechanically textured prior to application of any of the covering layers. Each of the underlay, magnetic layer and overcoat are typically applied by sputtering to provide a very even thickness. Accordingly, when the underlayer, magnetic layer and overcoat are applied over a textured substrate, the texturing is transmitted through each of these layers, and the resultant surface of the overcoat layer retains most of the texturing of the substrate.
Magnetic discs in a computer disc drive which use an air bearing slider have two discrete zones which may be defined based on how the slider of the magnetic head assembly travels over the disc surface. A landing zone is the zone where the slider containing the read/write transducer lands, rests while the disc drive is off, and takes off from when the disc drive is started up. A data zone is the zone where the slider flies over the disc and stores magnetic data. Lubricant is generally applied to both the landing zone and the data zone on the magnetic disc.
When the power is switched on in a disc drive, enough force has to be applied to the slider to overcome the static friction or "stiction" force on the slider. During the relatively low speeds ecountered during takeoff and landing, the slider maintains almost constant contact with the disc. Lubricant over the landing zone is important for its contribution toward the stiction force and to minimize wear and drag force during takeoff and landing.
As the speed of the disc increases, the slider bears off the air in contact with the disc surface such that the magnetic head assembly becomes airborne. During use of the disc drive, the magnetic head assembly is designed to fly over the disc surface without contacting the disc. Occasionally, however, the magnetic head assembly contacts the disc during use of the disc drive. These in-flight contacts between the slider and the media, although infrequent, occur at high speeds. Most of these contacts are caused by collision of the slider with media asperities, third bodies such as corrosion products, or other contaminant particles. Lubricant on the data zone can minimize wear and damage to the disc caused by occasional contacts between the magnetic head assembly and the disc.
With thin film magnetic media, minimal spacing between the head and the media is essential for high density storage. As flying altitudes are decreased, the tribology between the slider and disc surface becomes more and more important. Typical read/write heads now fly over the disc surface at altitudes in the 10-30 nm range. As the flying height becomes increasingly low, more and more physical and chemical interactions take place between the head and the disc during flying.
One of these interactions is the gradual transfer of the liquid lubricant from the disc surface to the head. This lubricant can accumulate anywhere on the head slider, but it is usually confined near the trailing edge or in the cavity. When the slider comes to a rest after power off, this lubricant can migrate back to the head/disc gap, and the flooding of this interface creates a very high adhesive force between the head and the disc. This force is known as stiction force, and the phenomenon is referred to as fly/stiction. Fly/stiction can lead the disc drive to a spin-up failure, as the motor may not be strong enough to overcome the high stiction force.
While the tribology between a slider and a disc is a function of the properties of the substrate and all the deposited layers, the overcoat and the lubricant are of primary importance. The slider structure also greatly affects the tribology, and sliders are usually formed of fairly hard ceramics such as Mn--Zn ferrite, calcium titanate (CaTiO.sub.3) and Al.sub.2 O.sub.3 --TiC.
The amount of stiction is a function of storage time as well as the normal force between the slider and the disc. The time dependency associated with the storage time is accredited to lubricant migration toward points of contact between the slider and the disc surface, resulting in meniscus forces which increase slowly over time. The time dependency of stiction may also be the result of increased elastic deformation of contacting asperities between the slider and disc surface, as well as the result of slow diffusion of ambient species, mainly water, into the lubricated junctions followed by displacement of lubricant from the meniscus.
One strategy to overcome the stiction problem has been to apply a light abrasive texture over the data zone, and a second roughness over the landing zone. This strategy is described in U.S. patent application Ser. No. 08/702,325, filed Aug. 22, 1996. A key to this thin-film media design is a light mechanical texture with a roughness average Ra greater than about 20 .ANG., as measured with an Atomic Force Microscope. However, as the flying altitude of the head continues to decrease, such a roughness on the disc surface becomes very undesirable because it can cause head-disc interference during the disc drive operation, leading to errors or even losses to the data.
The invention disclosed herein avoids the need for applying roughness to the disc surface by providing a lubricant comprising a narrow, high molecular weight fraction PFPE lubricant.